Use of photographic film for obtaining x-ray images has several disadvantages. The image is not immediately available because of the need to develop the film. Radiation exposure of the subject is high and exposure time is prolonged as a majority of the x-rays do not react with the film. Fluoroscopic screens enable instant viewing of an image but are otherwise subject to many of the disadvantages of film.
Efforts to resolve the problems associated with older x-ray imaging techniques have included use of an image intensifier and video camera imaging chain to generate a visible image on the screen of a display monitor. This produces a third generation image which tends to be degraded by electronic noise. The first generation image appears on a fluorescent a screen at the input of the image intensifier and the second generation image appears at another fluorescent screen at the output of the intensifier. The third generation image is produced by a video camera that views the image intensifier output. In order to improve image quality, the electronic signal generated by the image intensifier has been digitized to enable computerized image enhancement but this produces only marginal improvement.
In some more recent systems, the image intensifier system is replaced with an array of minute electronic x-ray detectors such as charge coupled devices. Data for constructing the image is read out of the array on a pixel by pixel basis to provide an image which may be displayed at the screen of a video display monitor. Primary disadvantages of these systems include high cost and complexity and an undesirably small field of view.
All of the prior x-ray imaging systems discussed above use what may be termed conventional geometry. That is, the x-rays diverge from a small fixed point and are detected at a large area detector such as the film, screen or detector array. My prior U.S. Pat. No. 3,949,229 issued Apr. 6, 1976 and entitled "X-ray Scanning Method and Apparatus" discloses an advantageous imaging system having a reversed geometry. The system of that prior patent uses an x-ray source having an extensive anode plate which is raster scanned by an electron beam to provide a moving x-ray origin point. X-rays emitted from different successive locations on the large anode plate in the course of a raster scan converge at an electronic detector which has a relatively small x-ray sensitive area. A moving light origin point at the screen of a display monitor undergoes a similar raster scan and is modulated by the detector output signal to provide the x-ray image at an analog X-Y storage cathode ray tube component of the monitor.
The reversed geometry provides a number of advantages. Radiation exposure of the subject may be greatly reduced as the electronic detector responds to incoming x-rays much more efficiently than film or a fluoroscopic screen. Collimators of the type disclosed in my prior U.S. Pat. No. 4,465,540 issued Aug. 14, 1984 and entitled "Method of Manufacture of Laminate Radiation Collimator" may be used to suppress x-rays that are not directed towards the small detector and which are therefore incapable of contributing to the desired image. The system can also be relatively uncomplicated and inexpensive in comparison with other forms of x-ray scanning equipment.
The reverse geometry also enables magnification of an area of the image that is of particular interest without relative movement of the subject, x-ray source and detection means. This is accomplished by reducing the size of the raster pattern at the anode plate of the x-ray source without making a corresponding reduction in the size of the raster pattern at the image display monitor. Conventional geometry systems require repositioning of the subject and/or the source and detector in order to accomplish a similar result. Magnification without such repositioning in a conventional geometry system reduces resolution in the image.
Initiating such magnification in the reverse geometry system of prior U.S. Pat. No. 3,949,229 is a somewhat time consuming and involved operation as a series of different controls must be manually adjusted and operator coordination of the adjustments with each other is necessary. Varying other characteristics of the image and changing operating parameters of the scanning x-ray source also require operator coordination of various manual controls and can be time consuming and somewhat taxing. Analog controls of this kind do not enable a number of highly advantageous modes of operation that will hereinafter be described.
Reducing the size of the raster scan area at the x.-ray source to obtain a magnified image concentrates electron beam heating at a limited area of the anode plate. Avoiding heat damage to the x-ray source requires careful attention by the operator and still more control adjustments.
My prior U.S. Pat. No. 4,259,582, issued Mar. 31, 1981 and entitled "Plural Image Signal System for Scanning x-Ray Apparatus", discloses reverse geometry scanning x-ray apparatus of the above discussed kind which as an option enables digitizing of the detector output and sweep frequency signals and digital storage of data values from which the detector output voltage and the raster scan sweep frequency voltages can be reconstructed in order to reproduce the x-ray image at a later time. The system further enables certain forms of digital processing of the data to change characteristics of the image. This includes magnification of a selected area of the image but does not provide for increased resolution or definition in the magnified region of the image. Control of the x-ray source and scan raster parameters continues to require time consuming adjustments and coordination of various analog voltage controls on the part of the operator.
The present invention is directed to overcoming one or more of the problems discussed above.